It is well-known to employ multi-screens for separating oversize and undersize impurities from a quantity of dry particles such as feed grain. U.S. Pat. No. 3,321,079 issued to Sackett, Sr. is believed typical of such prior art multi-screen devices. The main drawback of the Sackett device is a tendency for particles to clog one or more of the screens, effectively preventing the screens from filtering undesirable impurities from the main flow of particles.
In order to prevent the screens from becoming clogged with particles, it has been suggested in U.S. Pat. No. 1,644,537 issued to Meacham that scraper fingers sweep the vertically upper side of such screens. While such fingers can break up some clogs of particles extending upwardly from the screen, clogs of particles which become embedded in the screen openings are generally not disturbed by such fingers. The number of screen openings available for filtering undersize particles are thereby significantly reduced, resulting in a final product having an unacceptably large percentage of undersize particles and/or impurities.
A further approach to preventing the screens from clogging is suggested in U.S. Pat. No. 323,099 issued to Backus, wherein scraper fingers sweep beneath the screen to dislodge clogs extending downwardly from the screen openings. As with the Meacham device, the scraper fingers in Backus often fail to contact and dislodge clogs of particles embedded within the screen. As a result, undersize grain cannot pass through a significant number of the screen openings as desired.